Conventionally, 13Cr-type martensitic stainless steel pipes have been used in oil and/or gas well environments containing carbon dioxide gas, and are also standardized by the API (American Petroleum Institute). However, the 13Cr-type oil country tubular goods based on API standards (hereinafter referred to as API-13Cr oil country tubular goods) have deteriorated in toughness. In case of general API-13Cr oil country tubular goods particularly, the higher the strength, the more serious the deterioration in toughness. Therefore, the API-13Cr oil country tubular goods have been mostly used as oil country tubular goods for 85 ksi grade (yield strength: 85 to 100 ksi (552 to 689 MPa)) or less, with few known cases of extensive use as high-strength oil country tubular goods for 95 ksi grade with a yield strength (hereinafter also referred to as YS) of 95 to 120 ksi (656 to 827 MPa) or more grade.
Therefore, under the present circumstances, in response to a request for a higher-strength 13Cr oil country tubular goods for 95 ksi grade or more in oil and/or gas well environments containing carbon dioxide gas, the toughness is ensured by using a expensive material which is so-called “super 13Cr” which contains elements such as Ni, Mo and the like.
The oil country tubular goods, using the said “super 13Cr” as material, have excellent corrosion resistance in an environment containing carbon dioxide gas and trace of hydrogen sulfide in addition to satisfactory toughness. Therefore, in a case where only carbon dioxide gas corrosion resistance, high strength and satisfactory toughness are to be ensured, namely, sulfide cracking resistance is not required, there is a great request for using material, which is cheaper than the “super 13Cr”, for the oil country tubular goods.
Further, in marine oil and/or gas wells, there is a tendency to require high-strength steel pipes whose weight can be reduced by thinning, without changing the total strength in order to minimize the mass as much as possible, considering production and transportation costs. And also from an economical viewpoint, inexpensive higher-strength 13Cr oil country tubular goods are in demand as alternatives to the oil country tubular goods using the “super 13Cr” as material.
In the actual situation, however, general API-13Cr oil country tubular goods are hardly put into practical use as high-strength oil country tubular goods due to their inferiority in toughness as described above even though they have lower material cost.
Therefore, techniques for enhancing the toughness of API-13Cr oil country tubular goods by reducing the content of P to less than 0.010% by mass are proposed in Patent Documents 1 and 2.
Patent Document 1: Japanese Patent Laid-Open No. 11-310822
Patent Document 2: Japanese Patent Laid-Open No. 2001-323339